Miniature sensor chip, especially for finger print sensors

ABSTRACT

A sensor chip for measuring of structures in a finger surface includes an electronic chip provided with a number of sensor electrodes for capacitance measurements. The chip further includes a first layer comprising a metal or another electrically conducting material over and coupled to the sensor electrodes and a first dielectric layer substantially covering the first metal layer.

CONTINUITY

This application is a continuation of U.S. application Ser. No.10/297,745 filed Apr. 28, 2003, which is a 371 national phase ofPCT/NO01/00239 filed Jun. 8, 2001, which claims priority to NO 20003003filed Jun. 9, 2000, the respective disclosures of which are herebyincorporated by reference.

BACKGROUND AND SUMMARY

This invention relates to a sensor chip, especially for measuringstructures in a finger surface.

Fingerprint sensors based on capacitive coupling of an AC signal fromthe groove pattern of the finger to an array (matrix) of sensor elementsare known from international patent application No PCT/NO98/00182. U.S.Pat. No. 5,963,679 describes a similar sensor with a different measuringprinciple and a two dimensional sensor matrix.

For special applications, e.g. when being mounted in a cellular phone ora lap-top PC, it is important that the sensor is made as small and lightas possible. Such a miniature sensor may also be very cost effective ifproduction techniques suitable for mass production are used.

Noise considerations are important when positioning the electronicinterrogation circuit, often a costumer specified silicon circuit(“ASIC”) as close to the capacitance sensitive elements in the sensorarray as possible, so that the length of the electrical conductorsbetween the silicon circuit and the sensor elements is minimized.

The abovementioned requirements related to size and electronicfunctionality may be difficult to realize using standard packingtechniques in which the silicon circuit is mounted in a plastic based orceramic housing. At the same time as the housing effectively protectsthe sensor from the outer influences, such a solution may result in asensor with relative large dimensions, while the housing “legs” gives anunsuitable interface to the finger.

In addition to these general problems it is strongly desirable toprovide components adding extra functionality to the interface towardthe user, to secure optimal signal quality. Thus it would be a greatadvantage if such components may be integrated into the sensor itself.

The object of this invention is to secure a cost effective andminiaturized sensor solution based on a naked silicon chip (ASIC) with amulti layer metallization having a surface defining the interface towardthe user's fingers. The principle is based the fact that the array ofimaging sensor elements is positioned in one of the upper metal layerson a silicon surface.

In a standard production process for ASICs, the silicon surface isprovided with a number of metal layers with conductor leads, separatedby thin dielectric layers. In addition there may be built a so calledsandwich structure with extra layers of metal and dielectrics on top ofthe other layers. By laying the functional structures for detection ofthe finger's pattern in the upper layers of the ASIC, a miniature sensoras described above may be produced.

In U.S. Pat. Nos. 6,069,970; 5,862,248; and 5,963,679 fingerprintsensors are described being based on the abovementioned techniques.These solutions do, however, differ substantially in construction, sothat the functions of each layer are different. These solutions relateto very complex two dimensional sensor structures in which the impedanceis measured locally with drive electrodes for each pixel. This is,however, making strict requirements to the electronic circuitry ingeneral.

The present invention relates to a simplified solution to the challengesdescribed above.

Using a stimulation electrode in electrical contact with the finger animpedance measurement is obtained through the finger and the surface toa sensor electrode. This, in addition to the substantially linear sensorstructure, thus provides a largely simplified sensor relative to theknown solutions which is simple to produce and implement in small,mobile apparatuses.

Since the sensor thus may be produced using standard processes for ICproduction, this will give a very cost efficient sensor being suitablefor mass production.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described below with reference to the encloseddrawings, illustrating the invention by way of example.

FIG. 1 shows a cross section of the layered construction of metal anddielectric layers on the upper side of the sensor (schematically).

FIG. 2 shows a perspective drawing of the upper side of the sensor andof how the sensor chip may be mounted in a package (schematically).

FIG. 3 shows a sketch of the sensor point distribution according to apreferred embodiment of the invention.

FIG. 4 shows a combined modulation electrode and activation cell forpositioning on the sensor chip according to the invention.

DETAILED DESCRIPTION

As is evident from the drawings the sensor chip according to theinvention preferably consists of a naked silicon chip 6 in a housing 7without a cover (or possibly on a circuit board or ceramic substrate)with a number of layers of metal or another electrically conductivematerial such as polysilicon, and a dielectric constituting theinterface toward the user's fingers. The principle is based on imagingsensor elements constituting a part of the third or fourth upper layerof the electrically conductive layers 1 on the silicon circuit, and thatthe conductor lead, from the elements then is routed individually downto the amplifier circuits on the surface of the silicon chip.

The present solution may be produced industrially by standard methodsfor ASIC production and packing, as the silicon chip is mounted in aceramic or plastic based housing without a lid, or on a circuit board ora ceramic substrate. The silicon chip is coupled electrically to thecircuit board or housing with so called wire bonding.

In order to provide the necessary functionality for the capacitancebased measuring principle the three or four upper metal layers on theASIC must be used for different structures with a given electricfunction as for example illustrated in FIG. 1. The figure shows apossible solution is not limiting relative to other combinations oflayers.

The layers provided on the substrate shown in FIGS. 1 and 2 showexamples of the construction of the ASIC's upper layers with metal anddielectric, respectively, and will be explained below. The process forrealizing the metal and dielectric layers on the silicon chip is wellknown technology, but the functionality of the individual layers and thecombination of these is covered by the present invention, especiallyaimed at fingerprint sensors, as a variant of the sensor described ininternational patent application No. PCT/NO98/00182 which comprises anessentially linear sensor array 8, as shown in FIGS 2. and 3. FIGS. 2and 3 also show secondary sensor groups 9 which may be used for velocitycalculations, as described in the abovementioned patent. A line shapedsensor has the great advantage that it needs far less room and lessindividual channels than a two dimensional sensor with the sameresolution, so that it may be made at a much lower cost.

In FIG. 1 the electrical layer 1 constitutes the imaging sensor elements8. As mentioned the sensor elements may be positioned as shown in FIG.3, as a variant of the patented solution in international patentapplication No PCT/NO98/00182.

The dielectric layer 2, in FIG. 1, functions as an insulation layerbetween the conductor leads in the electrically conductive layer 1 andthe earth plane in the electrically conductive layer 3. The layer alsoconstitutes a part of the total dielectric thickness (insulation)between the finger and the sensor element.

The electrically conductive layer 3 constitutes an earthed planeshielding the conductor leads etc. against direct coupling of AC signalfrom the finger and from the modulation ring 5. The earthed layer 3 willpreferably be shaped so as to cover all conductor leads on theunderlying first electrically conductive layer 1, but must have windowsover the sensor elements 8. The openings in the earth plane 3 over thesensor elements 8 contributes with a “lens” effect to shape the electricfield and thus the capacitive coupling between the finger and the sensorelements, and the shape of the sensor element 8 and the opening in thesecond electrically conductive layer 3 may be optimized to maximize thesignal strength without going on accord with the geometric resolution ofthe sensor.

Alternatively the earthed shielding structure may be made using twoelectrically conductive layers being insulated from each other by adielectric layer. This will provide an improved shielding and lenseffect compared to the use of one layer.

The second dielectric layer 4 may insulate between the earth plane 3 andouter electrically conductive layer 5, and the second dielectric layer 4functions as an AC coupling of the signal to the electronics. The layer4 also constitutes (as mentioned above) a part of the total dielectricthickness (insulation) between the finger and the sensor element 8.Since the outer electrically conductive layer 5 of electrical reasonsdoes not cover the area over the sensor elements 8 the finger will comeinto contact with the second dielectric layer 4. Thus, this layer mustbe hard and wear resistant to withstand wear and breakage, as well aschemical influences from the surrounding environment and from the user'sfingers and other objects. This may be obtained using hard, impenetrabledielectrics such as Si₃N₄ and SiO₂.

The first electrically conductive layer 1 may in some cases beconstituted by a coupling between a separate electronic circuit and thefirst dielectric layer, so that they may be produced separately. Thesetwo parts may be produced with so called flip-chip technology. Inaddition, the other electrically conductive layer of the chip shown inFIG. 1 comprises a lower electrically conductive layer 11 making acommon earth plane for the circuit.

The outer electrically conductive layer 5 is the top layer having thepurpose of providing a stimulation signal, as mentioned in internationalpatent application No. PCT/NO98/00182, into the finger for securing thesignal quality, and as illustrated in FIG. 4. This therefore must beelectrically conductive with good coupling to the finger. It must alsobe hard and wear resistant to withstand wear and breakage, as well aschemical influences from the surroundings and the user's fingers. Chromeis a possible metal for this purpose.

Preferably the outer electrical conductive layer 5 is coupled to a drivecircuit for controlling frequency and amplitude of the stimulationsignal.

Such an stimulation electrode may be combined with for example acapacitive activation cell being capable of detecting when a fingertouches the sensor, and thus be used to control the activation of thesensor from a hibernation mode to active use, to minimize the powerconsumption of the sensor.

An impedance activation cell may e.g. be made as an interdigitatedfinger structure consisting of two not mutually interconnected “camstructures” (electrodes) 10 being shown in FIG. 4. When a conductiveobject, e.g. a finger, comes close to this structure the impedancebetween the electrodes will increase, and this change may be detectede.g. by an oscillating circuit operating with low power consumption.

If one of the electrodes is coupled to earth as long as it is inhibernation mode it will provide effective protection from ESDdischarges from a finger or some other charged objects close to thesensor, as the discharge will pass directly to earth. Even if one ofthese electrodes is not coupled to earth the structures in the outerelectrically conductive layer 10 will have an important function for ESDprotection if ESD protecting circuits are coupled between the structuresin this layer and earth.

An alternative method for coupling of a stimulation frequency into thefinger is to cover the conductive material with a thin dielectric filmso that the coupling is made purely capacitive. This may have theadvantage that the coupling is more equal from person to person andbeing independent of the dampness of the finger.

As the characteristics of the amplifiers and other signal treatmentelectronics on the chip will vary from element to element, it may be ofgreat importance to be able to calibrate the response from each sensorelement. This may be obtained using a transversal electrode close to theline of sensor elements or the conductors leading to them, e.g. as partof the earthed layer 3 or another electrically conductive layer belowit. By providing a calibration signal on this electrode the sensorelements will be excited capacitively without the presence of a fingeror other electrically conductive object close to the sensor. Based onthe resulting signals from the amplifier and signal treatmentelectronics it will then be possible to equalize the response from eachsensor element.

The present invention consists of a device which in a unique wayprovides the required advantages for a solid state fingerprint sensor.Corresponding technology may also be used in other applicationsrequiring recognition of finger movements over a surface, e.g. fornavigation/mouse purposes.

1. A sensor chip for measuring structures in a finger surface, thesensor chip comprising an array of sensor elements and a silicon chipcomprising electronic circuitry, including a number of interrogationelectrodes for capacitance measurements, said sensor chip furthercomprising: a first electrically conductive layer including said sensorelements defining positions of sensors in the sensor chip, said sensorelements being coupled to the interrogation electrodes in the siliconchip; a first dielectric layer substantially covering the firstelectrically conductive layer; a second electrically conductive layer onthe first dielectric layer, the second conductive layer includingopenings formed therein over the sensor elements, said secondelectrically conductive layer constituting an earth plane; a seconddielectric layer formed on the second electrically conductive layer; andan outer electrically conductive layer mounted on the second dielectriclayer for electric coupling with structures in the finger and notcovering the area over the sensor elements, wherein the silicon chip isadapted to measure the capacitance between the outer electricallyconductive layer and the interrogation electrodes.
 2. The sensor chipaccording to claim 1, wherein the first electrically conductive layer iscapacitively coupled to the interrogation electrodes.
 3. The sensor chipaccording to claim 1, wherein the outer electronically conductive layeris coupled to the silicon chip for application of a varying current orvoltage to the layer.
 4. The sensor chip according to claim 1, whereinthe outer electrically conductive layer is coupled to a drive circuitfor controlling the frequency and amplitude of a stimulation signal ofthe outer electrically conductive layer.
 5. The sensor chip according toclaim 1, wherein the second electrically conductive layer constitutes anessentially continuous layer with openings defined over the sensorelectrodes in the first electrically conductive layer.
 6. The sensorchip according to claim 1, wherein the thickness of the first and seconddielectric layers is equal to or less than the distance between thecenters of the sensor positions in the first electrically conductivelayer.
 7. The sensor chip according to claim 1, further comprising alower electrically conductive layer positioned under the silicon chip.8. The sensor chip according to claim 1, wherein the outer electricallyconductive layer is adapted to have contact with the finger and iscoupled to an electrical modulator in the silicon chip.
 9. The sensorchip according to claim 1, wherein the outer electrically conductivelayer comprises a structure for changing impedance when coming close toan electrically conductive object, and wherein the sensor furthercomprises circuitry adapted to cooperate with the structure for changingimpedance to activate the sensor when the surface to be measuredapproaches.
 10. The sensor chip according to claim 1, wherein thesensors form an essentially linear array.
 11. The sensor chip accordingto claim 1, wherein said array of sensor elements is an essentiallylinear sensor array with a secondary sensor group for measuring fingervelocity.